1. Field of the Invention
The present invention relates to a printing technique using a computer, and more particularly to a printing system which is capable of meeting a variety of needs of users. Means for generating pulse-width data for expressing a multiplicity of gray scales is incorporated in the printing system so as to supply the pulse-width data to a printing engine of a toner-transfer-type printing apparatus, such as a laser printer or a copying machine in the printing system.
2. Description of the Related Art
A toner-transfer-type printing apparatus is able to print a multi-gray-scale image by adjusting the pulse width of a drive pulse signal for driving drawing laser in a printing engine because the quantity of toner for each pixel to be allowed to adhere can be changed. Since the minimum size of toner particles however is limited, the number of gray scale levels, which can be expressed, is reduced in inverse proportion to the resolution of an image required to be printed. An assumption is made that image data having a resolution of 600 dpi and 256 gray scale levels is created on a personal computer and image data above is printed by a laser printer having a printing resolution of 600 dpi. One pixel having a resolution of 600 dpi is in the form of a square region, one side of which is 1/600 inch. The number of toner particles allowed to adhere to the square region is limited to about 20 to 30. Therefore, the image printed by the laser printer is able to reproduce at most 20 to 30 gray scale levels.
As described above, the conventional toner-transfer-type printing apparatus suffers from a problem in that the apparatus cannot reproduce a number of gray scale levels on a print output therefrom even if image data expressing the number of gray scale levels exceeding the number of toner particles allowed to adhere to one pixel region is supplied.
By the way, a usual printing system incorporates a digital printer and a host computer connected to the digital computer. The host computer has a printer driver installed thereon. The printer driver produces a print command issued to the printer to print an object which must be printed (hereinafter called an "object"). The printer receives the print command from the host computer so as to form an image of the object on a printing medium, such as paper. The digital printer is usually adapted to a binary printing method with which whether or not a small spot (dot) of a coloring material is formed is used to form an image.
The object includes plural types of objects, such as text data, graphic data and a natural image and the like. In this specification, the text basically means a characters described with data in a character code (text) format or a set of the characters. The graphics are in the form of so-called line drawing or a graphic image. The graphics mean an image which can be described with data in a vector graphic format (for example, data describing a function determining outline paths, the thickness of each outline and whether or not areas included in the outlines are filled in). The natural image is typified by a photograph having a substantially continuous gradation. The natural image means an image which can be expressed by only data (data formed by sets of pixel values indicating the density of each pixel or the gradation of the same) in the form which is called a raster image or a bit map image.
When a document is printed by a usual printing system, original data of the document is finally converted into a binary raster image which determines whether or not a dot of the coloring material is formed at the position of each pixel in a page of the document. The conversion includes an image drawing process (also called a rasterizing or rendering) and a half toning (also called a binary-coding) process. The image drawing process is a process for converting a text or vector graphic into a raster image. The half toning process is a process for converting a raster image having a multiplicity of gradations into a binary raster image so as to express the multiple gradations in a pseudo manner.
The usual printing system has two image drawing steps. That is, an intermediate code is produced for each band obtained by dividing one page, and the produced intermediate codes are stored in a memory (hereinafter called an "intermediate coding step"). The intermediate code is formed by listing sets of the raster image of each object (for example, each character, each figure or each photograph) and information about the size of the raster image and the position of the raster image in the page. Then, the raster image of the overall band is completely formed from the intermediate code of each band immediately before the band is printed so as to expand the raster image on the memory (hereinafter called "band expansion").
Specific methods of the foregoing image drawing and half toning processes and the share of the operation which must be performed by the printer driver and the printer vary for each system (that is, depending on the type of the printer). Also the share of the intermediate coding step and the band expansion step between the printer driver and the printer varies depending on the type of the printer.
A variety of needs for the printing operation are emerged from users. For example, a user sometimes requires a high speed printing operation, while a user sometimes requires a high-quality printing operation. Moreover, there is a requirement for faithfully printing an image, which cannot be drawn by an image-drawing device provided for the printer, as the image is observed on a display unit of a host computer (so called "What You See Is What You Get" abbreviated to "WYS/WYG").
Also a requirement from a user differs depending on the type of an object contained in a document. For example, a text or a graphic image is usually required to be printed such that the edges are clearly expressed. On the other hand, a natural image is required to be printed in such a manner that the gradation and the multiplicity of colors are maintained. If different objects overlap one another, there is a requirement for faithfully reproducing the logical relationship (for example, "and", "or" and "xor" hereinafter called a raster operation (ROP)) of the overlapping portions.
The conventional printing systems, however, cannot meet the above-mentioned various requirements. Although for example, a system incorporating a certain laser printer is able to clearly and quickly print text or graphic, the system has unsatisfactory performance to reproduce the image with a high quality.